1. Field of the Invention
This invention relates to circuits for receivers, to receivers, to systems having such receivers, and to methods of processing received signals. The present invention relates particularly to circuits for wireless receivers, to wireless receivers, to systems having such wireless receivers, and to methods of processing received radio signals.
2. Description of the Related Art
Bluetooth is a well known wireless protocol that describes how had held devices of II kinds such a PDA's, smart phones, mobile phones, computers, laptops, palmtops, peripherals and other devices can interconnect using a short-range wireless connection. The specifications of the Bluetooth System, v1.0 B, Dec. 1, 1999, and later versions, such as v1.2 and v2.0, are publicly available, and the reader is referred to these documents for more details. Bluetooth is a packet based frequency hopping protocol. A receiver typically has signal processing functions for waveform demodulation, DC compensation, bit synchronization and bit detection. Waveform demodulation usually is implemented in receive path of a radio receive module and is wireless protocol dependent. DC compensation can be implemented either in the receive module or in a subsequent baseband processing part. Bit synchronization and detection are usually implemented in the baseband and are common to many different wireless receivers. All Bluetooth data is transmitted as part of a packet. Standard rate packets are made up of four sections:
Access Code—The receiving device uses this to recognise incoming transmissions. It is a series of symbols to identify the start of a radio packet. It has a fixed length. In a normal communication mode, the master and slave units use the Master ID as the Access Code. As there is only one master unit in a network, the Master ID identifies the network.
Header—Describes the packet type and length.
Payload—The actual data.
Inter-Packet Guard Band—to enable retuning to the next frequency.
In the Bluetooth 1.2 specification, the access code, header and payload are modulated using Gaussian frequency-shift keying (GFSK) to modulate the over-air RF signal. In GFSK, the carrier frequency deviates by +/−160 kHz to indicate a one or a zero thus encoding one bit per symbol. The symbol rate is 1 MSymbol/s, leading to a peak data rate of 1 Mbit/s. But, when designers account for access codes, headers, and guard bands, Bluetooth systems can deliver a maximum payload data rate of 723 kbit/s.
To provide a higher data rate, the EDR (Enhanced Data Rate) protocol defines additional packet types having new modulation schemes for payload data. In the Bluetooth 2.0 specification, EDR packets use GFSK modulation for the access code and header, and one of two different modulation schemes for the payload. The change of modulation scheme also requires the insertion of a small guard band and a synchronization word between the header and the payload. EDR data uses π/4 differential quaternary phase-shift keying (π/4-DQPSK), or eight-phase differential phase-shift keying (8DPSK), both of which involve altering the phase of the carrier rather than the frequency.
The receiving radio needs to be able to switch modulation schemes between the header and the payload. A Bluetooth packet-header contains 4 bits for packet identification, and messages can be exchanged so that EDR compatible radios can switch between modes. from 1 Mbit/s, to 2- or 3-Mbit/s modes. This is typically all handled by lower Bluetooth stack layers embedded into chips, and below the HCI interface used by higher level application layers.
In a known transmitter-receiver device (WO 00/18150) for a digital communications system a radio signal received from an aerial is demodulated in the receiver circuit in order to generate a digital input signal. This digital input signal is supplied to a synchronization circuit, which compares the input signal with an expected access code, in order to accept the data packet if the received access code is identical to the expected access code. Otherwise the received input signal is rejected. In order to enable a bit by bit comparison between a digital input signal and an expected access code or synchronization word, the corresponding bit value is allocated to each symbol in the input signal corresponding to a bit in the input signal in that the signal level of the input signal for each symbol is compared with a threshold value, corresponding to an estimated or expected d.c. voltage quota.
US patent application 2002048330 shows a device for identifying a data packet in a data stream, in a Bluetooth receiver for example. The d.c. voltage quota for a demodulated digital input signal is calculated, a k-bit word is allocated to the input signal, in that for each symbol of the input signal corresponding to a bit a bit value of 1 or 0 is determined as a function of the d.c. voltage quota, the k-bit word corresponding to the input signal is compared with an expected k-bit synchronization word. This provides a correlation value and a packet identification signal is generated if the correlation value is greater than a correlation threshold value.